Sunlight is an abundant component of the natural environment. Sunlight may photo-degrade environmental pollutants to less toxic chemicals, or may generate more reactive and toxic species. Often the light energy determines what kind of species is produced. UV light can break chemical bonds forming free radicals while less energetic visible light may induce photosensitization processes via electron and/or energy transfer producing reactive oxygen species. We are interested in the (photo) reactions involving free radicals and/or singlet oxygen of nitrite, nitrate, nitric oxides NOx and humic acids.We have studied the UVA-induced production of NO and OH radicals from nitrite. These radicals may subsequently react with many organic or inorganic substrates. UV radiation also generates free radicals in aquatic humic matter. We have studied the humics from different sources such as rivers, ocean, and soil-based samples, finding that their photochemical properties differ quite considerably. Photosensitized oxidation processes are importance in the natural environment because of the abundance of sunlight and the presence of natural and pollutant photosensitizers. We have examined the photochemical properties of water samples from selected ponds in Minnesota and Vermont in which frogs with unexplained malformations have been found. It has been suggested that such malformations may somehow be related to the increase in UVB irradiation reaching the earths surface as a result of the decrease in stratospheric ozone. A water sample from an affected Minnesota pond generated larger amounts of singlet oxygen upon UV irradiation than water from a pond where no frog anomalies had been observed. However, this observation did not hold for the Vermont samples. Water samples from other affected areas are currently being examined. Although nitrite is generally considered as the end product of NO generation in vivo, we have found that, in the presence of H2O2, lactoperoxidase, metabolizes nitrite to a strongly oxidizing species, probably the NO2 radical. The latter can react with biological electron donors such as NADH, NADPH, cysteine, glutathione, and ascorbate thereby causing oxidative stress. - toxic chemicals, UV light, organic substrate, inorganic substrate